One of the methods for manufacturing a photodiode array in semiconductor materials with narrow band gap (often for detection in infrared light)—is to insert the active detection layer with narrow band gap between two semiconductor materials having a wide band gap. The two layers of wide band gap semiconductors provide efficient protection/passivation whilst remaining transparent to the radiation wavelength intended to be detected by the photodiodes.
In addition, with suitable doping, the two heterojunctions between the active layer and the two protection/passivation layers confine the photoelectric charges within the active detection layer and improve the quantum yield of the photodiode thus constructed.
An InGaAs photodiode is a typical example of this physical structure. The active detection layer formed of InGaAs material can have an adjustable band gap as a function of the indium and gallium composition of the InGaAs material, ideal for operation in the SWIR band (Short Wave InfraRed) in the order of 1.4 to 3 μm.
Indium phosphide and indium-gallium arsenide share the same face-centred cubic crystalline structure. The composition most frequently used is In0.53Ga0.47As. The size of the crystalline lattice is then comparable with that of the InP substrate, in particular the lattice parameters. This crystalline compatibility allows epitaxial growth of an active InGaAs layer of excellent quality on an InP substrate. The band gap of In0.53Ga0.47As is about 0.73 eV, capable of detecting up to a wavelength of 1.68 μm in the SWIR band. It is of increasing interest in fields of application such as spectrometry, night vision, the sorting of waste plastics, etc.
The two protection/passivation layers are generally formed of InP. More especially since the In0.53Ga0.47As composition has the same size of crystalline lattice as InP this allows for a very low dark current on and after ambient temperature.
FIG. 1 illustrates the physical structure of a photodiode array 1. An active layer composed of InGaAs is sandwiched between two InP layers. The bottom layer in fact forms the substrate 4 on which the InGaAs layer is formed by complex MO-CVD epitaxy. This InGaAs layer is then protected by a thin passivation layer 6 composed of InP, also deposited by epitaxy. In general, the InP layers are of N-type and silicon-doped. The active InGaAs layer 5 can be slightly N-doped or it can remain quasi-intrinsic. Therefore the two bottom/top InP layers and the active layer 5 of InGaAs form the common cathode of the photodiodes in this array.
The individual anodes 3 are formed by local diffusion of zinc (Zn). The dopant Zn passes through the thin InP passivation layer 6 and enters into the active InGaAs layer 5.
FIG. 2 illustrates an InGaAs image sensor formed of an array 1 of InGaAs photodiodes connected in flip-chip mode to a read-out circuit 2. In an InGaAs sensor array the array of photodiodes is connected to a read-out circuit generally formed of silicon to read the photoelectric signals generated by these InGaAs photodiodes. This interconnection is generally obtained using the flip-chip technique via indium beads 8 as illustrated in FIG. 2. SWIR radiation 9 arrives at the array of photodiodes through the substrate 4 of indium phosphide that is transparent in this optical band.
With a detector operating in integration mode, an output signal is obtained proportional to the product of flow and exposure time. However, the output signal is limited by the maximum integration capacity of the detector. For scenes having high contrast, it is often impossible to obtain good rendering of dark areas and at the same time to maintain the bright areas saturation-free. This is all the more problematic for night vision for which a sensor array with InGaAs photodiodes is often intended.
Another general manner for reading the photoelectric signals of photodiodes is put forward in document EP1354360 and for which the principle is illustrated in FIG. 3 of the appended drawings. Document EP1354360 proposes the solar cell functioning of a photodiode 51 to obtain a logarithmic response as a function of the intensity of incident optical radiation 59.
With this functioning mode, the photodiode 51 does not receive any external biasing and is forward biased by the photoelectric charges generated at its junction. The forward bias voltage observed on the photodiode is proportional to the logarithm of incident optical flow.
This logarithmic response allows the coverage, without any electrical and optical adjustment, of a dynamic operating range greater than 120 dB, indispensable for use of an InGaAs SWIR sensor under natural outside conditions. Document EP1354360 also proposes associating a switched read-out circuit 55 with the photodiode.
The principle of use of the image sensor array illustrated in FIG. 3 is the following:                a) the selection signal SEL is activated to select the desired photodiode 51 by closing switch 54. Once this photodiode is selected, the first read-out signal RD1 is activated which will close the corresponding controlled switch for the purpose of memorising the voltages of a first read-out in the memory 56. This first read-out records both the image and the fixed pattern noise;        b) The reset signal RSI is then activated which will cause closing of switch 53. The photodiode being thus short-circuited, a reference image is thereby simulated in absolute darkness;        c) The first read-out signal RD1 is then deactivated to re-open the corresponding switch and the second read-out signal RD2 is activated to record in the memory element 57 the voltages of the second read-out. In this manner the fixed pattern noise alone has been memorised;        d) the difference is calculated between the result of the two memorisations contained in the memory elements 56 and 57 respectively by a differential amplifier 58. The output signal of this amplifier 58 then corresponds to an image free of fixed pattern noise.        
By means of the second read-out, a zero voltage corresponding to the darkness condition is generated. This electronic darkness signal allows the deletion of signal offsets in the read-out chain of a detector array.
The principle proposed by EP1354360 was applied in an InGaAs sensor and operates perfectly. However a blooming phenomenon was observed in daytime scenes. This phenomenon can simply be described as loss of spatial resolution in an image. The detector nevertheless remains sensitive to variations in light following the logarithmic law.
French patent application N° 1156290 proposes electric insulation by etching around each photodiode. With this approach it is possible to obtain efficient suppression of this blooming phenomenon but at the cost of a very strong dark current in the photodiodes due to defects created by this etching. Another problem with this approach is the fact that the steps of etching and photodiode anode diffusion are two separate steps in the manufacturing process requiring different masks. Mask alignment errors can create additional non-uniformities between the photodiodes of an array.